Hi, I have a Dali E9F subwoofer in my 5.1 HT setup. Mostly satisfied with its performance but sometimes, at not so loud volumes, I see it going nuts with the cone visibly vibrating vigorously and sound clearly distorted in some movie scenes. What could be the issue here? It is being driven by Denon x1500h AVR and I used Audessey to calibrate. Do I need to manually set its level even lower or am I just playing it too loud (even though other speakers don't sound as loud)? How should I fix it? Please advice. Thanks in advance!